Method and a device for inductive heating of work pieces



Oct; 2, 1962 A. SCHMIDT 3,056,876

METHOD AND A DEVICE FOR INDUCTIVE HEATING OF WORK PIECES Filed Dec. 5, 1958 INVENTOR AR THUR SCHMID T United States Patent 3,056,876 METHOD AND A DEVICE FOR INDUCTIVE HEATING OF WORK PIECES Arthur Schmidt, Reichenbach (Fils), Germany, assignor to North American Philips Company, Inc., New York,

N.Y., a corporation of Delaware Filed Dec. 5, 1958, Ser. No. 778,460 Claims priority, application Germany Dec. 24, 1957 11 Claims. (Cl. 219-1041) The present invention relates to a method and apparatus for inductive heating. More particularly, the invention relates to a method and apparatus suitable for carrying out this method for inductive heating of Work pieces of highly different wall thicknesses and sectional area in the direction of length at the desired uniform temperatures. These work pieces may be, for example tubular work pieces with comparatively thick bottoms, such as cartridge cases, shell cases and the like.

It is known to manufacture these work pieces in a manner such that first a cup is pressed from a blank or a rod stub. The cup is subsequently drawn out by various processes in the axial direction of length until the desired dimensions are attained. This method has been practised for a long time with many variations in the manufacture of sheet iron material and other parts deformed in the cold state.

With cold deformation the rigidity of the metal worked thereby increases, which is to be compensated for intermediate between the various drawing operations. This is carried out by annealing at low temperature, by the so-called recrystallisation annealing process, and which is absolutely required for most of the conventional raw materials such as steel, brass, copper and other light metal.

This annealing at low temperatures was carried out in annealing furnaces. Annealing furnaces occupy a large space and which have various further disadvantages in modern manufacturing methods, so that nowadays the use of inductive heating also for recrystallisation annealing processes is steadily growing.

In an annealing furnace, the material traverse the heating zone, which is kept at the desired temperature, and is brought to the desired temperature mainly by convection and radiation during its travel, the work piece being kept at this temperature for some time, if required.

With inductive heating, the material traverses an induction coil, an induction loop or a similar device, which produces the required heat inductively in th Work piece. In this case the power transmitted per centimetre of length is constant, apart from certain peripheral effects at the beginning and at the end of the coil; it is due thereto that a solid, circular body assumes the desired temperature much more slowly than a thinwalled body. Since the induction coil is energised by comparatively high frequencies, skin-eifect is also to be taken into account, which effect is operative in the same direction. This often gives rise to diiliculties with inductive heating of drawn and pressed work pieces, the important advantages of inductive heating being thus reduced, or the use thereof being even rendered impossible.

The aforementioned ditficulties arise particularly in the manufacture of cartridge cases, since these cases comprise a comparatively long, thin-walled part with a wall thickness of about 1 to 2 mms. and a bottom of about 15 mms. and more in thickness.

When such a cartridge case, which may b a semiproduct, is introduced into a uniformly wound induction coil, i.e. an induction coil having a constant distance between the turns, the cylindrical portion is heated very rapidly, whereas the bottom remains comparatively cold. It may be considered advantageous to reduce slightly the Patented Oct. 2, 1962 ice total energy and to prolong accordingly the heating time in order to obtain a uniform distribution of heat by conduction inside the case. However, this reduces materially the efiiciency of the device, whilst it is almost impossible to ensure a uniform heating.

It may be considered to reduce this drawback with Work pieces of irregular siz by providing an irregular pitch in the induction coil. In this case, the area of an increasing size would require a correspondingly increasing number of turns in the coil. Since the power produced is approximately proportional to the square of the number of turns per coil centimetre and per centimetre of length of the work piece respectively, differences in wall thickness could be compensated for Within certain limits.

With this method it is, of course, disadvantageous that only a single work piece can be heated in one coil at a time. In the case of a larger number of work pieces per hour, a correspondingly larger number of induction coils have been provided.

However, it is a particularly troublesome disadvantage that in the case of very thick bottoms or in the case of large sections having thin walls as compared therewith, the differences in the ampere-turns become so high that the manufacture of suitable coils becomes impossible. This applies particularly to cartridge cases of larger gauge.

Another possibility of overcoming these difficulties may entail the building of long induction coils, through which the separate work pieces are moved with their base or mouth parts in contact with each other or in the same direction, i.e. each of the work pieces are aligned to move with the base of one piece is behind the mouth of another work piece, one after the other. In this case, the coil must have a smaller pitch at the area of the base or bottom than at the other areas. Apart from the fact that also in this case the differences in the required ampere turns may be too great, there is the condition that also the lengths of the cases become critical and should be accurately the same in order to ensure a correct positioning of the bottoms inside the coil. Any variation in case length would require a corresponding variation in pitch. It is thus not possible to move work pieces of different lengths through the same coil, which may be of great importance for manufacture, since as is stated above, as the drawing processes are carried out between the annealing processes, the lengths of the work pieces are varied.

The present invention mitigates these difficulties and in accordance with the invention the heating of the thickwalled part of the work piece is carried out by using an individual inductor, apart from the heating of the thinwalled part, both in space and in time. The thin-walled part of the work piece is heated by another inductor.

Thus, the periods and the required powers for heating the thickwalled portions of the work piece and for the thinwalled portions are independent of one another. It is thus possible to heat the thickwalled bottom of a case with high power within a given period and to heat subsequently the thinwalled cylinder with a different power within a different period of time. First the bottom is heated and then the cylinder together with the bottom is heated.

When, as is mostly the case in practice, comparatively large numbers of work pieces per hour are to be heated, it is efiicient, in accordance with the preferred embodimeut of the invention, to move the work pieces in a regular order of succession through a loopor coil-shaped inductor. The coil-shaped inductor acts only upon the thickwalled portions of the work pieces and heats a plurality of work pieces, so that the thickwalled portions of the work pieces are preheated at a value lying below the final temperature. Then the work pieces are brought separately, in the same apparatus, at the same rate, to

another inductor, which acts upon the whole work piece and which heats the preheated portion of the work piece simultaneously with the other part thereof at the final temperature.

This method provides heating of the thickwalled portions of the work pieces and of the preheated, complete work pieces in a manner such that the desired final temperature is uniformly divided in the whole work piece. As the weight of the thickwalled portion of the work piece becomes greater with respect to the total weight of the work piece, it is advantageous, in practicing the teachings of this invention, to increase the duration of the action of the loop or coiled shaped inductor accordingly.

Also, in utilizing this method, since a plurality of work pieces are moved through an elongated, loop-shaped inductor which acts simultaneously upon the whole work pieces, it may be advantageous to rotate the work pieces during the conveyance, at least in the loopor coilshaped inductor, about an axis at right angles to the direction of travel. This rotation improves the uniformity of the heating obtained, since the distances between the different points of the work piece surfaces from the heating turns of the inductor are not constant. This method may, of course, also be used, when the preheated work piece is supplied to an inductor for final heating, where non-uniform distances between the conductors and the work piece prevail.

Apparatus for carrying out the method according to the invention may comprise a conneyance system which moves the work pieces with their thickwalled portions through an elongated induction loop, which heats a plurality of work pieces at the same time. The whole Work pieces are then heated in another inductor.

Since the work pieces may be conveyed rectilinearly in accordance with the structure of the conveyance sys- .tem employed, the introduction of the work pieces into the work space of the elongated inductor may give rise to difficulties. It is therefore eflicient to provide a bend of the elongated induction loop at least on one side, so that even with a rectilinear conveyance such difliculties cannot arise.

Another way to overcome this difficulty entails the use of thrusting members, for example studs or bolts movable in their direction of length, by means of which the work piece can be moved toward or away from the conveyance system on one side. The thrusting member provided at the end of the conveyance system may then be utilized also to supply the preheated work piece delivered' by the conveyance system to the second inductor.

A rotating disc may serve as a conveyance system. The edge of the disc may be provided with suitable members for holding the work pieces. The disc may move, if desired stepwise, the work piece through the elongated inductor to the second inductor, which produces the final heating. it is advantageous to utilize a chain as a conveyance system. Holders for the work pieces may be provided on the chain to move the work pieces with their thick-walled portions through the working space of the elongated inductor.

It is then advantageous to feed the work pieces to the conveyance system by means of a feeder which delivers them separately under the control of the drive of the conveyance system in the correct order of succession to the collecting member of the conveyance system. The term correct order of succession is to be understood .to mean herein that, for example in the case of a thinwalled case with a thick bottom, these work pieces are fed to the chain so that the bottoms are located on the same side of the chain. The first part of the conveyance system or, as the case may be, also the feeder of the work pieces, then have preferably lateral guides, which orientate thework pieces with respect to the conveyance system or the elongated inductor. It is thus ensured that the thickwalled portions of the work pieces are accurately located whole.

at the same spot relative to the elongated inductor, which is then capable of heating them with maximum efficiency.

The drawing shows one embodiment of the invention.

FIG. 1 is a side view of a device for inductive heating of cases with thick bottoms.

FIG. 2 is a plan view of the same device.

vFIG. 3 shows a temperature diagram of a method according to the invention.

The device shown in FIGS. 1 and 2 for inductive heating of work pieces comprises a chain conveyor 1 for conveying the work pieces. The conveyor comprises mainly the two chain wheels 2 and 3 and a sprocket chain belt 4. The drive of the chain conveyor 1 is omitted for the sake of simplicity.

The chain conveyor 4 is provided with holders 5 for the work pieces 6, which, in the embodiment shown, consist of a thinwalled case provided at one end with a thick bottom or base. The work pieces are'fed to the chain conveyor '1 by way of a gutter serving as a feeder 7, of which the cross section is U-shaped and of which the upright sides 14 orientate the work pieces relative to one another with the required accuracy. It is assumed that the work pieces are supplied in the correct position, i.e. with the bottoms facing forward as shown in FIG. 1, on the gutter. tin order to deliver the separate work pieces to the holders 5 of the chain 4, the end of the gutter is provided with a gear mechanism 8, which is coupled with the driving of the chain conveyor and causes at the correct instant a work piece to glide onto the next-following, idle holders 5. As shown in P16. 1, the chain wheels 2, 3 move clockwise, so that the holders 5 are moved always from the left side to the lower end of the gutter 7.

Alongside the upper portion of the chain conveyor is provided an elongated induction loop 9, which comprises in the embodiment shown in FIG. 2 three turns arranged closely side by side. The front end thereof is bent at right angles to the front at 10, so that the thick ends of the work pieces, i.e. the thick bottoms, can enter unhindered the working space of the induction loop 9. After the work pieces thick bottoms have traversed the induction loop 9, they attain, at a short distance before the closed end of the induction loop 9, a position in which each work piece individually is in line with an ejector 11.

This ejector slides the work piece laterally from its holder 5; the work piece then attains the working space of a second inductor 12, in this case a coil-shaped inductor and illustrated in FIG. 2 as being behind the conveyance system. The work piece is supported by a suitable holder, which is formed in the embodiment shown by a gutter 13. The work piece is subjected throughout its length to the action of the inductor and is therefore heated as a When the ejector 11 introduces the next-following work piece, the work piece heated in the inductor 12 is pushed away by the next-following work piece and is carried to the next-following position for further machining.

The size of the two inductors 9 and 12 and the highfrequency currents used are chosen so that the work piece is heated as illustrated in FIG. 3. In the curve of FIG. 3, the temperature T is plotted against the time t. The time t is the instant when a work piece enters the loop-shaped inductor 9. The inductor 9 acts mainly upon the thick bottom of the work piece and during the conveyance of the work piece through the elongated inductor the bottom temperature rises according to the first section of the curve to the temperature T The heating takes a period of time and the temperature T is attained at the instant t At this instant the work piece is located in front of the ejector 11. It is carried away from the working space of the inductor 9 and attains the interior of the inductor 12. This transport takes some time, so that the work piece reaches the interior of the inductor 12 at the instant 1 During this intermediate period the bottom is not further heated, so that its temperature drops to the value T Since the inductor 12 acts upon both i the bottom and the wall, or the rest of the work piece, and has only low power, the temperature in the bottom rises slowly from the instant 1 whereas the temperature of the rest of the work piece rises rather abruptly from the instant 1: This is indicated by the last section of the curve. At the instant t the cylinder, or wall, of the work piece and its bottom have the same temperature T in this case the desired final temperature. The work piece can then be removed from the inductor 12. If this removal is to take place in the aforesaid manner by means of the ejector 11 with the aid of the next-following work piece, the conveyance rate and the power to be transmitted by the two inductors can be easily adjusted accordingly. This may be accomplished, for example, by choosing the period of time t t to be equal to the period of time t t and by adjusting the current intensities in accordance with the dimensions of the bottom and of the whole work piece respectively.

As briefly stated above, various variants of such an inductive heating system are possible. Instead of using a chain conveyor 1, use may be made, for example, of a rotating disc. An ejector similar to the ejector 11 may serve also for introducing the Work pieces into the Working space of the elongated inductor, so that the bend of the inductor 9 at the left-hand end 10 may be dispensed with. The inductor 9 may have bends at both ends in cases, for example, Where an ejector 11 is not utilized and the supply and the delivery of the work pieces take place along a straight line to bath inductors 9 and 12 with the second inductor 12 being formed similar to coil 9 either to cover all of or only that portion of the work piece containing the thinwalled case and arranged at the side of the conveyance system, instead of being positioned behind it, as was aforedescribed hereinabove and illustrated in FIG. 2, or if a pause is desired on the basis of the heating curve suitable for individual cases, and so on. It is obvious to those skilled in the art, the invention may be modified so that only the thinwalled part of the work piece will be heated by the subsequent inductor 12 with or without appropriate changes in the inductor 12 and/ or heating curves. Thus, While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as a limitation to the scope of my invention as set forth in the accompanying claims.

I claim:

1. A method for inductively heating a Work piece to a substantially uniform desired temperature, said work piece having first and second portions disposed in respective mutually exclusive positions along a predetermined axis thereof and having first and second thicknesses in respective planes substantially normal to said axis, said first thickness being greater than said second thickness, comprising the steps of inductively heating at a first station, the said first portion of said work piece with a first inductor constructed and arranged to heat substantially only the said first portion, and inductively heating at a second station at least said second portion of said work piece with a second inductor energized independently of said first inductor thereby to produce a predetermined temperature distribution between said first and second portions of said Work piece.

2. A method for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising the steps of moving said work pieces in a regular order of succession through a substantially loop-shaped first inductor adapted to heat substantially only said first portion of each of said work pieces, inductively preheating said first portions of said work pieces in said first inductor to a temperature below said desired temperature, moving said work pieces separately in said order of succession at the same rate into a second inductor energized independently of said first inductor and adapted to heat at least said second portions of said work pieces, and inductively heating said second portions of said work pieces in said second inductor to said desired temperature.

3. A method for inductively heating a plurality of work pieces per a predetermined interval of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising the steps of moving said work pieces in a regular order of succession through a substantially loop-shaped first inductor adapted to heat substantially only said first portion of each of said work pieces, inductively preheating said first portions of said work pieces in said first inductor for a predetermined period of time to a temperature below said desired temperature, moving said work pieces separately in said order of succession at the same rate into a second inductor energized independently of said first inductor and adapted to heat at least said second portions of said work pieces and inductively heating said second portions of said work pieces in said second inductor for a period of time shorter than said predetermined period to said desired temperature, said predetermined period of time being longer in duration relative to said last-mentioned period of time in accordance With the weight said first portion has with respect to the weight of said whole work piece.

4. A method for inductively heating a plurality of work pieces per a predetermined interval of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising the steps of moving said work pieces in a regular order of succession through a substantially loop-shaped first inductor adapted to heat substantially only said first portion of each of said work pieces, rotating each of said work pieces about an axis at right angles to the direction of motion of the said work pieces during the motion through said first inductor, inductively preheating said first portions of said work pieces in said first inductor for a predetermined period of time to a temperature below said desired temperature, moving said work pieces separately in said order of succession at the same rate into a second inductor energized independently of said first inductor and adapted to heat at least said second portions of said work pieces, and inductively heating said second portions of said work pieces in said second inductor for a period of time shorter than said predetermined period to said desired temperature, said predetermined period of time being longer in duration relative to said last-mentioned period of time in accordance with the weight said first portion has with respect to the weight of said whole work piece.

5. Apparatus for inductively heating a work piece to a substantially uniform desired temperature, said work piece having first and second portions disposed in respective mutually exclusive positions along a predetermined axis thereof and having first and second thicknesses in respective planes substantially normal to said axis, said first thickness being greater than said second thickness, comprising first inductor means at a first station constructed and arranged to inductively heat substantially only said first portion of said work piece, and second inductor means at a second station for inductively heating substantially at least said second portion of said work piece, said second inductor means being energized independently of said first inductor means thereby to produce a predetermined temperature distribution between said first and second portions of said work piece.

6. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially at least uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising substantially loop-shaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, conveyance means moving said work pieces in a regular order of succession through said firstinductor means, said first portions of said work pieces being inductively preheated in said first inductor means to a temperature below said desired temperature, second inductor means for inductively heating substantially said second portions of said work pieces, said second inductor means being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

7. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising substantially loopshaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, said first inductor means comprising a loop having a bent configuration at one end thereof adapted to facilitate entry of said work pieces into said loop, conveyance means moving said work pieces in a regular order of succession through said first inductor means, said first portions of said work pieces being inductively preheated in said first inductor means to a temperature below said desired temperature, second inductor means for inductively heating substantially at least said second portions of said work pieces, said second inductor means being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

8. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising substantially loopshaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, conveyance means moving said work pieces in a regular order of succession through said first inductor means, said first portions of said work pieces being inductively preheated in said first inductor means to a temperature below said desired temperature, second inductor means for inductively heating substantially at least said second portions of said work pieces, said second inductor means being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said last-mentioned moving means comprising a thrusting member positioned in proximity to said conveyance means in a manner whereby each said work piece may be moved off the said conveyance means into said second inductor means, said second portions of said work pieces being inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

9. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising substantially loopshaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, said first inductor means comprising a loop having a bent configuration at one end thereof adapted to facilitate entry of said work pieces into said loop, conveyance means moving said work pieces in a regular order of succession through said first inductor means, said conveyance means comprising a chain conveyor and a plurality of holders carried by said chain, each of said holders being adapted to hold a Work piece in a manner whereby said first portions of said work pieces are moved through said first inductor means, said first portions of said work pieces being inductively preheated in said first inductor means to a temperature below said desired temperature, second inductor means for inductively heating substantially at least said second portions of said work pieces, said second inductor means being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

10. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said Work piece, comprising substantially loop-shaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, said first inductor means comprising a loop having a bent configuration at one end thereof adapted to facilitate entry of said work pieces into said loop, conveyance means moving said work pieces in a regular order of succession through said first inductor means, said conveyance means comprising a chain conveyor, means driving said chain conveyor and a plurality of holders carried by said chain, each of said holders being adapted to hold a work piece in a manner whereby said first portions of said work pieces are moved through said first inductor means, means controlled by said driving means for feeding said work pieces to said holders individually and in a position suitable for entry into said loop, said first portions of said work pieces being inductively preheated in said first inductor means to a temperature below said desired temperature, second inductor means for inductively heating substantially at least said second portions of said Work pieces, said second inductor means being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

11. Apparatus for inductively heating a plurality of work pieces per predetermined period of time to a substantially uniform desired temperature, each of said work pieces having a first wall portion thicker than a second portion of the said work piece, comprising substantially loop-shaped first inductor means for inductively heating substantially only said first portion of each of said work pieces, said first inductor means comprising a loop having a bent configuration at one end thereof adapted to facilitate entry of said work pieces into said loop, conveyance means moving said work pieces in a regular order of succession through said first inductor means, said conveyance means comprising a chain conveyor, means driving said chain conveyor and a plurality of holders carried by said chain, each of said holders being adapted to hold 'a work piece in a manner whereby said first portions of said work pieces are moved through said first inductor pieces, said second inductor mean being energized independently of said first inductor means, and means moving said work pieces separately in said order of succession at the same rate into said second inductor means, said second portions of said work pieces being inductively heated in said second inductor means to said desired temperature.

References Cited in the file of this patent UNITED STATES PATENTS Goodridge Jan. 12, 1943 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,056 876 October 2 1962 Arthur Schmidt It is hereby certified that error appears in the above numbered pat-- ent requiring correction and that the said Letters Patent should read as corrected below.

Column 7 line .8 after "substantially" insert at least lines 60 and 61 strike out "said second portions of said work pieces being inductor meansfl',

Signed and sealed this 25th.day of June 1963.

(SEAL) Attest:

ERNEST w. SWIDER DAVID L. D Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,056.,876 October 2 1962 Arthur Schmidt It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should vread as corrected below.

Column] line 8 after "substantially" insert at least lines 60 and 61 strike out "said second portions of said work pieces being inductor meansfi'q Signed and sealed this 25thday of June 1963.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer I Commissioner of Patents 

